Epidemic typhus is characterized by iramatic changes in vascular tone and permeability, and in hemostasis. The results of these changes are hypotension decreased blood volume, tissue anoxia, and shock due to peripheral vascular collapse. The reasons for these changes are not understood: the key may lie in understanding the implications of rickettsial interactions with their target cells. Although virulent typhus rickettsiae may survive within unstimulated macrophages, they grow primarily within endothelial cells. This is significant because endothelial cells directly regulate or secrete products that regulate vascular tone and permeability, thrombosis, platelet aggregation, immune responsiveness and leukocyte margination. The general purposes of this study will be to infect monolayers of human endothelial cells with typhus rickettsiae, to monitor the multiplication of the rickettsiae and viability of the monolayer, and determine the relative ability of infected endothelial monolayers to express activities or products which effect vascular tone and permeability thrombosis, and platelet aggregation. To this end, rickettsia-infected and sham-infected endothelial cells will be incubated for up to seven days. At various times, cells from individual wells will be stained for rickettsiae and the number of rickettsiae per cell will be determined. Each sample will be examined for one of the following endothelial products by the method described: (1) tumor necrosis factor production (by ELISA), (2) interleukin-1B production (by ELISA), (3) platelet activating factor production (by thin layer chromatography), (4) endothelium-dependent protein C activation (by use of an amidolytic assay), (5) interferon-gamma production (by radioimmunoassay), and (6) prostaglandin T2 (PGI2) secretion (by radioimmunoassay). These products or activities are of particular interest because of the critical roles they play in regulation of vascular function. Because some of these processes are phospholipid dependent, additional experiments will examine effects of rickettsial entry into endothelial cells on these same activities. Endothelial cells will be exposed to high multiplicities of rickettsiae and examined for the activities described. Finally, rickettsia-infected endothelial cells will be cultured in the activities described. Finally, rickettsia-infected endothelial cells will be cultured in the presence of one of each of the effectors described above to determine their effect on rickettsial multiplication. These studies should allow greater understanding of the impact of rickettsial infection on endothelial function, and, ultimately, on the disease process in typhus.